In the production of gasoline, the desire to produce a clean product is constantly present. This desire comes both from increased environmental awareness and regulation and from a general desire to maximize product performance. In many hydrocarbon feedstocks commonly used to make gasoline via catalytic cracking, sulfur is present as an undesirable impurity.
In conventional fluidized catalytic cracking (FCC) operations, a portion of the sulfur may be removed via formation of H.sub.2 S during the cracking operation or by formation of sulfur-containing coke on the cracking catalyst particles. Unfortunately, the gasoline resulting from such FCC processes typically will still contain a significant amount of sulfur from the original feedstock.
Currently, if it is desired to reduce the sulfur content of the output gasoline, some additional treatment step has typically been necessary. For example, the feedstock may be treated before cracking in a separate step involving the use of Mn-containing compositions (U.S. Pat. No.2,618,586), Cu on inorganic oxide (U.S. Pat. No. 4,204,947), titania on clay (U.S. Pat. No. 4,549,958) or other substances. Alternatively, the sulfur content of output gasoline has been reduced via hydrotreatment of the feedstock. These known measures typically increase the refining cost both from the need for added equipment to perform the additional process steps and from the need to use additional materials in the refining process.
Recently, certain compositions have been developed which can be used directly in an FCC operation (i.e., in the circulating catalyst inventory) to reduce the sulfur content of the resulting gasoline without use of additional process steps or the use of added hydrogen. Such compositions, disclosed in U.S. Pat. No. 5,376,608, comprise an alumina-supported Lewis acid component. The disclosure of U.S. Pat. No. 5,376,608 is incorporated herein by reference.
While the compositions of U.S. Pat. No. 5,376,608 are effective, there is a desire to obtain an even greater degree of reduction in the output gasoline sulfur level from FCC processes without use of additional process steps or the use of added hydrogen.